Arc furnaces for refining include a D.C. arc furnace which causes a current to flow between an electrode disposed above metallic materials charged into the furnace and an electrode fitted to a furnace bottom or a furnace wall such as a side wall and which effects melting of the metallic materials and refines a molten metal, and an A.C. arc furnace which causes a current to flow through three electrodes disposed above the molten metal charged into the furnace, and effects melting of the metallic materials and refining of the molten metal. Exhaust gas utilization systems in the arc furnaces of these kinds generally have constructions wherein a pre-heating vessel is disposed between the arc furnace and dust collecting equipment, a bucket of the metallic materials is inserted into this pre-heating vessel and the metallic materials are pre-heated by an exhaust gas.
However, the exhaust gas duct between the arc furnace and the pre-heating vessel has a water-cooling structure because a high temperature gas generated by the arc furnace flows through this duct, and the temperature of the exhaust gas reaching the pre-heating vessel drops to an extent corresponding to the temperature drop due to water-cooling. Therefore, the retained heat of the exhaust gas cannot be efficiently utilized for pre-heating the metallic materials, and this is a critical problem from the aspect of heat efficiency. Further, the white smoke and the offensive odor that are generated when the pre-heated metallic materials are conveyed in the bucket to the arc furnace are also a critical problem from the aspect of the operating environment.
Therefore, various proposals have been made so far in order to efficiently utilize the heat of the exhaust gas for pre-heating the raw materials and to charge the pre-heated raw materials into the arc furnace without conveying them from the pre-heating vessel to the arc furnace. Japanese Unexamined Patent Publication (Kokai) No. 3-505625, for example, describes an arc furnace which pre-heats the raw materials in a pre-heating zone formed by a shaft disposed on a furnace cover and a part of a furnace body and lets the pre-heated raw materials, which are stored in the shaft, drop naturally.
In this case, however, after the raw materials stored in the shaft disposed on the furnace cover naturally drop and are then molten, or in other words, at the so-called "refining stage", the raw materials no longer exist inside the shaft, and are not pre-heated by the heat of the exhaust gas any more. Therefore, the problem of heat efficiency was still unsolved.
To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 5-500263, for example, proposes a method which disposes two sets of arc furnaces, introduces the exhaust gas into the other of the arc furnaces, which stores the raw materials inside the shaft and is in a stand-by state, so as to pre-heat the raw materials after the raw materials stored in one of the arc furnaces naturally drop and are molten by electric power supplied to this arc furnace, and pre-heats the raw materials by the other arc furnace so as to improve heat efficiency.
Japanese Unexamined Patent Publication (Kokai) No. 4-309789 proposes a method which forms the pre-heating zone in only the shaft, disposes a raw material holding gate inside the shaft, makes it possible to always store the raw materials inside the shaft and thus utilizes the heat of the exhaust gas at the refining stage.
By the way, a major portion of the energy for melting the metallic materials in the arc furnace is provided by electrical energy. When the energy efficiency for melting the metallic materials is taken into consideration, it is desirable to position the arc as the energy source as close as possible to the center portion of the furnace body, that is, to dispose the upper electrode to bring the charging portion of the metallic materials to be molten as close as possible to the center portion of the furnace body and to generate the arc from the upper electrode at the center of the metallic materials so as to uniformly melt them throughout the periphery.
This proposal exploits fully the great advantage of the D.C. arc furnace in that only one upper electrode is needed and the energy utilization efficiency is higher than the A.C. arc furnace which requires three upper electrodes. This advantage quite naturally holds true of the case where the metallic materials, as the object of melting, are pre-heated.
However, in the prior art proposals of Japanese Unexamined Patent Publications (Kokai) Nos. 3-505625, 5-500263 and 4-309789 described above, the shaft is disposed on the side wall portion of the furnace body and the pre-heated raw materials are therefore charged to only the side wall portion of the furnace body. In other words, the portion of the furnace body opposite to this side wall portion to which the raw materials are not charged is always heated uselessly by the arc heat and is not used for melting the raw materials, thereby inviting the drop of energy utilization efficiency. Since the raw materials are charged only to the portion of the furnace away from the upper electrode, the melting speed becomes lower than the case where melting is carried out by disposing the upper electrode above the center portion of the raw materials, and energy utilization efficiency drops.
To solve this problem, it may be conceivable to employ a method which charges the raw materials while the furnace cover and the shaft are moved so that the shaft comes to the center portion of the furnace body. In this case, the upper part of the furnace body is opened with the movement of the furnace cover in the furnace cover shape according to the prior art, and when the raw materials inside the shaft are charged into the furnace, a gas containing high temperature dust is blasted out from the upper portion of the furnace body and remarkably deteriorates the operation environment.
In an arc furnace wherein a shaft for pre-heating raw materials is disposed on a furnace cover, and this shaft and an upper electrode are positioned at the center of the furnace cover and capable of moving up and down are integrally disposed, Japanese Patent Application No. 6-144079 filed previously by the inventors of the present invention to solve the problem described above employs the construction wherein the furnace cover is movably constituted and the furnace cover covers the upper portion of the furnace body even at the time of movement.